Problem: On the first 3 quizzes of her chemistry class, Stephanie got an average score of 88. What does she need on the next quiz to have an overall average of 91?
Answer: Let her score on the next quiz be $x$ The sum of all of her scores is then $3 \cdot 88 + x$ The same sum must also be equal to $4 \cdot 91$ Solve: $x = 4 \cdot 91 - 3 \cdot 88 = 100$.